This invention relates to methods and apparatus for the production of fused silica optical members. More particularly, the invention relates to methods, burners and furnaces incorporating burners for the production of high purity fused silica.
As practiced commercially, fused silica optical members such as lenses, prisms, photomasks and windows, are typically manufactured from bulk pieces of fused silica made in large production furnaces. In overview, silicon-containing gas molecules are reacted in a flame to form silica soot particles. The soot particles are deposited on the hot surface such as a layer of bait sand of a rotating or oscillating body where they consolidate into to the glassy solid state. In the art, glass making procedures of this type are known as vapor phase hydrolysis/oxidation processes, or simply as flame hydrolysis processes. The bulk fused silica body formed by the deposition of fused silica particles is often referred to as a xe2x80x9cboule,xe2x80x9d and this terminology is used herein with the understanding that the term xe2x80x9cboulexe2x80x9d includes any silica-containing body formed by a flame hydrolysis process.
FIG. 1 shows a typical furnace 100 for producing fused silica glass. As shown in FIG. 1, furnace 100 includes a crown which carries plurality of burners which produce the silica soot which is collected on a collection surface 24 to form a boule 19, which typically has a diameter on the order of five feet (1.5 meters).
Boules typically having diameters on the order of five feet (1.5 meters) and thicknesses on the order of 5-10 inches (13-25 cm) can be routinely produced in large production furnaces of the type shown in FIG. 1. Multiple blanks are cut from such boules and used to make the various optical members referred to above. The optical axis of a lens element made from such a blank is generally parallel to the boule""s axis of rotation in the furnace. For ease of reference, this direction will be referred to as the xe2x80x9caxis 1xe2x80x9d or xe2x80x9cuse axisxe2x80x9d.
In the past, burners 14 have been unable to deposit soot in a sufficient manner at distances greater than six inches from the burner face, which has meant that the maximum boule thickness has been six inches. Commonly assigned PCT patent application publication number WO 00/17115, the entire contents of which are incorporated herein by reference, describes a burner capable of producing boules having a thickness of 8-10 inches. The burner in this PCT application includes concentric regions emitting the following gases: 1) a central region (fume tube) which emits a mixture of a halide free silicon containing raw material and an inert gas, 2) an inner shield gas region which emits oxygen, 3) a third region which emits a mixture (premix) of combustible gas and oxygen, 4) a fourth region which emits a mixture (premix) of combustible gas and oxygen, 5) a fifth region which emits a mixture (premix) of combustible gas and oxygen, and 6) a sixth region, which is an outershield region which emits oxygen.
As noted in PCT patent application publication number WO 00/17115, burners that work well in one particular application (e.g., optical waveguide performs or smaller sized boules), or starting material (e.g., halide free starting materials versus halide containing materials), may not be successful in another application having different operating conditions. In the course of development of a furnace for producing high transmission fused silica boules, applicants surprisingly discovered that the burner described in PCT application publication number WO 00/17115 did not work particularly well in producing high transmission glass. Applicants found that this burner did not provide sufficient heat to the target surface onto which the glass was being deposited. Applicants attempted to increase the gas flows through the burner described in the PCT application, however, the flame velocity increased proportionally, creating flames with excessive force which forced heat and unconsolidated soot away from the target surface back towards the crown of the furnace. Applicants further discovered that over time, this soot built up and eventually formed droplets of fused glass which drip down onto the surface of the boule, requiring the furnace to be shut down.
The next generation of fused silica glass used in the microlithography market will require ArF (193 nm) internal transmission exceeding 99.65%/cm, and preferably greater than 99.75%/cm. As noted above, during the course of development of a process and a furnace capable of manufacturing such high transmission glass, applicants discovered the need for a burner to produce more heat at a velocity that would not create flames that caused the problems noted above.
The invention relates to methods and apparatus for producing fused silica. According to one aspect of the invention, a method for producing silica-containing boule is provided, which includes providing a furnace including a burner for producing soot in a flame disposed above a collection surface, the burner including at least seven gas-emitting regions. According to this aspect, the method further involves providing a mixture of a carrier gas and a silicon-containing material to a first region, providing oxygen to the second region, providing a mixture of combustible gas and oxygen to the third, fourth, fifth and sixth regions, providing oxygen to the seventh region, and collecting soot on the collection surface to form the boule. In another aspect of the invention, the second region surrounds the first region, the third region surrounds the second region, the fourth region surrounds the third region, the fifth region surrounds the fourth region, the sixth region surrounds the fifth region and the seventh region surrounds the sixth region. According to a preferred aspect of the invention, the velocity of the flame is controlled to enhance the efficiency of the collection step. According to another aspect of the invention, the boule has a thickness greater than ten inches.
Another aspect of the invention involves consolidating the boule during the collection step. Preferably, the silicon-containing precursor is halide free. According to another aspect of the invention, the distance between the burner and the collection surface remains constant during formation of the boule.
Another aspect of the invention relates to a fused silica member produced in accordance with the method described above. The fused silica member preferably has an internal transmission of at least 99.65%/cm, and preferably of at least 99.75%/cm at 193 nm.
Another aspect of the invention relates to a soot-producing burner comprising a burner face including first, second, third, fourth, fifth, sixth and seventh gas-emitting regions, wherein the first region emits a mixture of silicon-containing material and a carrier gas, the second region emits oxygen, the third, fourth, fifth, and sixth regions emit a mixture of combustible gas and oxygen, and the seventh region emits oxygen. In a preferred aspect, each of the gas emitting regions are concentrically disposed about the burner face. In a particularly preferred aspect, the second region surrounds the first region, the third region surrounds the second region, the fourth region surrounds the third region, the fifth region surrounds the fourth region, the sixth region surrounds the fifth region, and the seventh region surrounds the sixth region.
Still another aspect of the invention relates to apparatus for producing silica-containing soot. The apparatus comprises a burner having a face including a first region for emitting a silicon containing precursor and a carrier gas, two oxygen regions for emitting oxygen, and four mixture regions for emitting a mixture of combustible gas and oxygen. According to this aspect, the first region is disposed in a central region of the face, a first oxygen region surrounds the first region, the four mixture regions surround the first oxygen region, and a second oxygen region surrounds the four mixture regions. The apparatus further includes a supply of silicon containing precursor in communication with the first region and a supply of oxygen in communication with the two oxygen regions and the four mixture regions.
The invention provides the advantage of providing a high heat capacity burner that produces a low velocity flame that improves the manufacturing process for fused silica boules. Additional advantages of the invention will be set forth in the following detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.